1. Field of the Invention
The present invention relates in general to exhaust gas purifying systems of an internal combustion engine, and more particularly to the exhaust gas purifying systems of a type that is equipped with a PM collecting filter that collects PM (viz., particulate matter) in the exhaust gas from the engine. More specifically, the present invention is concerned with a technique for timely reactivating the PM collecting filter.
2. Description of the Related Art
Hitherto, various exhaust gas purifying systems of the above-mentioned type have been proposed and put into practical use particularly in the field of diesel engines. One of them is described in Laid-open Japanese Application (Tokkaihei) 7-34853. In this system, a PM collecting filter is installed in an exhaust passage line of a diesel engine, and the filter is subjected to a heating for its reactivation at a given reactivation time. With the heating, the PM is burnt away and thus removed from the filter thereby to achieve a reactivation of the filter. For finding the reactivation time, a clogging degree of the filter is practically used, which controls the pressure loss at the position downstream of the filter. That is, under operation of the engine, exhaust pressures at positions upstream and downstream of the filter are monitored. By measuring a pressure difference between the two exhaust pressures, an amount of the PM collected by the filter is estimated, and when the estimated amount of the collected PM exceeds a predetermined value, it is judged that the reactivation time has come.